Electron beam columns are used in scanning electron microscopes (SEMs) that image objects and in lithography tools for writing patterns onto semiconductor materials to be used as integrated circuits. Conventional electron beam columns consist of an assembly of components, including lenses, magnets, deflectors, blankers, etc., individually machined out of stainless steel or other alloys and individually assembled, and an electron source.
Alternatively, miniature electron beam columns can be made by using, in part, micro-fabricated lenses, deflectors and blankers. These components are fabricated in silicon using micro-electromechanical systems (MEMS) fabrication technologies. Each component consists of vertically stacked silicon lenses that are electrically isolated by dielectric spacers, like, for example, glass. The silicon and glass elements have at least one aperture concentric with very other aperture creating a path for the electron beam to transverse. The components are energized to focus, blank, and steer the electron beam.
Focused electron, ion or photon beams are a proven technique of achieving extremely high-resolution, high-placement accuracy imaging, patterning and analysis. A major limitation of these techniques, however, is the serial manner in which each pixel on a work surface must be addressed, whether for writing or for imaging, which can result in unacceptably low throughputs. A well-known technique for increasing throughput linearly or quadraticly is to use linear or rectangular arrays of beams which operate in parallel. In this way, the resolution of the beam may be fully utilized without suffering unacceptable losses in wafer or sample throughput.
To this end, a number of parallel, distributed beam techniques have been introduced including miniature microfabricated beam columns (microcolumns), zone plate arrays (ZPAL), distributed variable/fixed aperture electron beam systems (DIVA/DIFA), and massively parallel beam systems (MAPPER). A common shortcoming of these systems however is the inability to accurately register (align) one beam to another and thereby meet the placement accuracy required in future generation lithography and inspection tools. Although a number of techniques have been proposed to address this issue, mostly based on marks on a wafer or substrate, these techniques are generally either too impractical, too time-consuming or too inaccurate for implementation in a production tool.
Miniature microfabricated beam columns are particularly amenable to parallel operation in linear or rectangular array by the nature of the MEMS processes used in fabrication and the small footprint of the column. The columns may be individually assembled or monolithically fabricated into linear or rectangular arrays and operated in parallel. The actual size and shape of the arrays are primarily dictated by the writing or reading strategy adopted at the system level. A major problem however, as discussed above for all distributed axis beam systems, is beam-to-beam calibration. Even with nanometer level placement of the lens apertures, variations in assembly tolerances, accuracy of tip placement, tip temperature, source alignment, etc., an extremely reliable and efficient system and method of beam to beam alignment is required.